QoS Question

[hakeemsimmons]

This could be a stupid question, but it has confused me quite a bit.

I have a layer 2 switch that has the "mls qos trust cos" statement on an access port.

I have a device that marks DSCP values as EF, or the ability to mark as TOS. Layer 3 markings.

This switch was forcing the DSCP markings to 0, as the switchport would only trust COS markings, this is in the 802.1p header.

In order for DSCP to pass, I needed to institue the "mls qos trust dscp" command instead of the "mls qos trust cos" it worked.

I guess I have two questions based on that knowledge above.

Is "dscp cs5" a COS marking? or is this simply a TOS 5 critical marking?

How can an IP phone mark COS before it goes accross the trunk port that would carry it at layer 2?

Some insight would really be appreciated.

Thanks.

[Jonathan Schulenberg]

How can an IP phone mark COS before it goes accross the trunk port that would carry it at layer 2?

The link between a Cisco IP phone and the switch is also an 802.1q trunk by virtue of using the voice VLAN. Thus the packets can be tagged with a COS header.

Is "dscp cs5" a COS marking? or is this simply a TOS 5 critical marking?

Yes, it is part of the IP header TOS field.

TOS and DSCP, which happen on the Layer3 header, are not the same as COS which is at Layer2. TOS provided a CS0-7 inclusive range of values. DSCP added three more bits to introduce far more granular marking.

More info is here: http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml

Don't get wrapped up around TOS vs. COS. Focus on whether the marking is occuring at Layer 2 or 3.

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